The present invention relates to a method for use in determining the permeability of rocks in a subterranean formation surrounding a borehole.
Permeability is a measure of the hydraulic conductivity of a rock, i.e., the ease or difficulty with which a fluid (oil, gas or water) flows through porous rock. In oil/gas fields, the permeability of the rock associated with an oil/gas reservoir is an important parameter to measure, because it is directly related to the rate of oil or gas that can be produced. To elaborate, in exploiting an oil/gas reservoir, one or more wells or boreholes are drilled at locations that typically have been determined based upon information obtained from seismic surveys, exploratory wells and the like. Once the wells have been drilled, the permeability of the rock formations encountered in the borehole is an important parameter in managing the well or wells so as to maximize the oil/gas that can be extracted from the reservoir.
There are a number of methods presently in use for determining the permeability of the rock in a borehole. One method, known as formation testing, involves using a wireline to lower an instrument to a desired location within the borehole. The instrument is adapted to force a nozzle into contact with the rock. The nozzle is then used to draw fluids from the rock. The ease or difficulty associated with drawing the fluids is indicative of the permeability of the rock. Formation testing, while providing a direct measurement of permeability, is subject to errors attributable to borehole fluid invasion effects and drilling induced formation damage.
Production testing is another method of directly measuring permeability. In production testing, a zone of a borehole is hydraulically isolated with packers and the fluid production of the well is guided to the surface through a sting of pipes. The volume of fluids produced and the thickness of the zone are used to derive the permeability of the rock in the isolated zone. Production testing, while typically providing reliable data, is very expensive due to the substantial amount of equipment required and the length of the test period, typically several days.
Yet another method involves the use of Stoneley acoustic waves to measure parameters that are related, but not equivalent to permeability. These measurements are then used to infer the permeability or range of permeabilities. Stoneley acoustic waves are waves that travel along the interface between the rock that defines the borehole and the fluid in the borehole. This method also uses a wireline to lower instrumentation into the borehole that causes Stoneley waves to be formed. The same tool detects the Stoneley waves after they have interacted with the rock of interest. Unlike formation testing, the instrumentation is not placed in direct contact with the rock. Consequently, the instrumentation is typically designed so that measurements are taken while the instrumentation is being raised or lowered by the wireline. One of the major shortcomings associated with the Stoneley waves approach is that the relationship of the measurements made with Stoneley waves to permeability can vary substantially from reservoir to reservoir. To address this variability, it is necessary to perform calibration procedures that make use of permeability measurements made on cores taken from the borehole or measurements made during formation testing.
Another method that provides an indirect measure of permeability utilizes nuclear magnetic resonance. To elaborate, nuclear magnetic resonance is used to measure the decay times of protons that are indicative of the pore size distribution, which in turn can be used to infer the permeability of the rock of interest. This method of measuring permeability is typically implemented using a moving wireline logging instrument. The limitations associated with nuclear magnetic resonance are substantially the same as those noted with respect to the Stoneley or tube wave approach.
The present invention is directed to a method of measuring the permeability of rock within a borehole that makes use of a slow compressional wave, which propagates through the rock of interest. To elaborate, an acoustic or wave that travels through a porous rock can manifest itself in three modes: (1) a fast compressional wave, which is also known as a p-wave, in which the solid rock material and the fluid in the pores move in phase with one another; (2) a slow compressional wave in which the solid rock material and fluid in the pores move out of phase with one another; and (3) a transverse wave, which is also known as a shear wave or s-wave, in which the particles move perpendicular to the wave propagation direction. The compressional wave presently known to be used in making borehole measurements is the fast compressional wave with a velocity of approximately 2000-3000 m/s. The present invention makes use of the slow compressional wave, which typically has a velocity of approximately 900 m/s.
The method involves transmitting a compressional wave into the rock of interest at a fixed location within the borehole. The compressional wave manifests itself both as fast compressional wave and a slow compressional wave. The slow compressional wave is subsequently detected at a second location that is a known distance from the first location. Information associated with the transmitting and detecting steps is subsequently used to determine the velocity of the slow compressional wave. Namely, the elapsed time between the transmission of the compressional wave and detection of the slow compressional wave and the known distance between the first and second locations provides sufficient information to determine the velocity of the slow compressional wave. The velocity of the slow compressional wave is, in turn, combined with other information to calculate the permeability of the rock of interest.
Among the other information that, in conjunction with the velocity of the slow compressional wave, is used to determine the rock permeability is the velocity of the fast compressional wave. In some instances, the velocity of the fast compressional wave may be available from prior measurements. However, since the transmission of the compressional wave used to produce the slow compressional wave also produces a fast compressional wave, one embodiment of the invention also detects the fast compressional wave. The velocity of the fast compressional wave is determined from the elapsed time between the transmission of the compressional wave and the detection of the fast compressional wave and the known distance between the location from which the compressional wave was transmitted and the location at which the fast compressional wave is detected.
In one embodiment, the locations associated with the transmission of the compressional wave and detection of the slow compressional wave are chosen to be between about 10 cm and 50 cm of one another because of the high degree of attenuation that is likely to be experienced by the slow compressional wave as it propagates through the rock of interest. Further separation of the two locations is feasible. However, more sensitive detectors are required and/or signal processing to separate the slow compressional wave signal from noise.
In one embodiment, the location from which the compressional wave is transmitted (first location) and the location at which the slow compressional wave is detected (second location) are chosen so that a permeability measurement is made in a preferred direction or dimension. In one situation, the first and second locations define a line that is substantially parallel to the longitudinal axis of the borehole. In a vertical borehole, this orientation of the first and second locations results in information being obtained that is useful in determining the vertical permeability of the rock of interest. In another embodiment, the first and second locations define a line that is substantially perpendicular to the longitudinal axis of the borehole. This orientation, in a vertical borehole, provides data that is useful in calculating the horizontal permeability of the rock of interest. In yet another embodiment, the slow compressional wave is detected at two locations that, together with the first location, define a right angle. In this instance, the orthogonal permeabilities of the rock of interest are determinable. In the case of a vertical borehole, this means that both vertical and horizontal permeabilities are determinable.